Fibre Channel Arbitrated Loop (FC-AL) architecture is a member of the Fibre Channel family of ANSI standard protocols. FC-AL is typically used for connecting together computer peripherals, in particular disk drives. The FC-AL architecture is described in NCITS working draft proposal, American National Standard for Information Technology “Fibre Channel Arbitrated Loop (FC-AL-2) Revision 7.0”, Apr. 1, 1999.
Electronic data systems can be interconnected using network communication systems. Area-wide networks and channels are two technologies that have been developed for computer network architectures. Area-wide networks (e.g. LANs and WANs) offer flexibility and relatively large distance capabilities. Channels, such as the Small Computer System Interface (SCSI), have been developed for high performance and reliability. Channels typically use dedicated short-distance connections between computers or between computers and peripherals.
Fibre Channel technology has been developed from optical point-to-point communication of two systems or a system and a subsystem. It has evolved to include electronic (non-optical) implementations and has the ability to connect many devices, including disk drives, in a relatively low-cost manner. This addition to the Fibre Channel specifications is called Fibre Channel Arbitrated Loop (FC-AL).
Fibre Channel technology consists of an integrated set of standards that defines new protocols for flexible information transfer using several interconnection topologies. Fibre Channel technology can be used to connect large amounts of disk storage to a server or cluster of servers. Compared to Small Computer Systems Interface (SCSI), Fibre Channel technology supports greater performance, scalability, availability, and distance for attaching storage systems to network servers.
Fibre Channel Arbitrated Loop (FC-AL) is a loop architecture as opposed to a bus architecture like SCSI. FC-AL is a serial interface, where data and control signals pass along a single path rather than moving in parallel across multiple conductors as is the case with SCSI. Serial interfaces have many advantages including: increased reliability due to point-to-point use in communications; dual-porting capability, so data can be transferred over two independent data paths, enhancing speed and reliability; and simplified cabling and increased connectivity which are important in multi-drive environments. As a direct disk attachment interface, FC-AL has greatly enhanced I/O performance.
Devices are connected to a FC-AL using hardware which is termed a “port”. A device which has connections for two loops has two ports or is “dual-ported”.
The operation of FC-AL involves a number of ports connected such that each port's transmitter is connected to the next port's receiver, and so on, forming a loop. Each port's receiver has an elasticity buffer that captures the incoming FC-AL frame or words and is then used to regenerate the FC-AL word as it is re-transmitted. This buffer exists to deal with slight clocking variations that occur. Each port receives a word, and then transmits that word to the next port, unless the port itself is the destination of that word, in which case it is consumed. The nature of FC-AL is therefore such that each intermediate port between the originating port and the destination port gets to ‘see’ each word as it passes around the FC-AL loop.
FC-AL architecture may be in the form of a single loop. Often two independent loops are used to connect the same devices in the form of dual loops. The aim of these loops is to provide an alternative path to devices on a loop should one loop fail. A single fault should not cause both loops to fail simultaneously. More than two loops can also be used.
FC-AL devices typically have two sets of connections allowing them to be attached to two FC-ALs. Thus, in a typical configuration, two independent loops exist and each device is physically connected to both loops. When the system is working optimally, there are two possible loops that can be used to access any dual-ported device.
A FC-AL can incorporate bypass circuits with the aim of making the FC-AL interface sufficiently robust to permit devices to be removed from the loop without interrupting throughput and sacrificing data integrity. If a disk drive fails, port bypass circuits attempt to route around the problem so all disk drives on the loop remain accessible. Without port bypass circuits a fault in any device will break the loop.
In dual loops, port bypass circuits are provided for each loop and these provide additional protection against faults. A port can be bypassed on one loop while remaining active on the dual loop.
A typical FC-AL may have one or two host bus adapters (HBA) and a set of six or so disk drive enclosures or drawers, each of which may contain a set of ten to sixteen disk drives. There is a physical cable connection between each enclosure and the HBA in the FC-AL. Also, there is a connection internal to the enclosure or drawer, between the cable connector and each disk drive in the enclosure or drawer, as well as other components within the enclosure or drawer, e.g. SES device (SCSI Enclosure Services node) or other enclosure services devices.
Components in a loop can be categorised as “initiators” or “targets”, or both depending on their function in the loop. For example, a host bus adapter is an initiator and a disk drive is a target. Initiators can arbitrate for a communication path in the loop and can choose a target. A target can request the transfer of a command, data, status, or other information to or from the initiator.
If there is a single initiator in a loop, the initiator will login with all the targets in the loop. Targets may accept or reject this login attempt. At any later stage a target can log out with any logged in initiator. In a multi-initiator environment, an initiator operates as both a sender and recipient login attempts.
When target devices such as disk drives are provided on dual loops with a port on each loop, such devices do not necessarily cope with being accessed by the same or by different initiators on both ports. This may even cause data transfer rates to be reduced because of the overhead in switching between ports. It is also possible that there are ordering issues to worry about.
There may be other advantages in only accessing target devices via one port, such as being able to bypass redundant ports. Therefore, accessing multi-ported targets via only one port is proposed.
Devices may not accept or correctly complete log in procedures and this is a problem if devices do not present the same view to each initiator.
Providing multiple initiators in a loop network should increase performance levels and achieve a higher degree of connectivity. Therefore, management of a loop network with more than one initiator is proposed. Accesses to dual-ported devices in a loop network should be balanced to evenly distribute the accesses to devices over both of the dual ports. This needs to be co-ordinated with the proposal to access devices through only one port for all initiators.
It is an aim of the present invention to provide management of a loop network with more than one initiator to provide a consistent view of the devices in the loop network and to balance the accesses to the devices.